Illumination system particularly for microlithography

ABSTRACT

There is provided an illumination system for microlithography with wavelengths ≦193 nm. The illumination system includes a primary light source, a first optical component, a second optical component, an image plane, and an exit pupil. The first optical component transforms the primary light source into a plurality of secondary light sources that are imaged by the second optical component in the exit pupil. The first optical component includes a first optical element having a plurality of first raster elements that are imaged into the image plane, producing a plurality of images being superimposed at least partially on a field in the image plane. The plurality of first raster elements are rectangular. The field is a segment of an annulus, and the second optical component includes a first field mirror with negative optical power for shaping the field to the segment of the annulus and a second field mirror with positive optical power. Each of a plurality of rays intersects the first field mirror with an incidence angle greater than 70° and each of the plurality of rays intersects the second field mirror with an incidence angle of less than 25°.

REFERENCE TO RELATED APPLICATIONS

The present application is (a) a U.S. national stage entry ofInternational Application No. PCT/EP01/11232, and (b) a continuation-inpart of U.S. patent application Ser. No. 10/201,652. The PCT/EP01/11232application was filed Sep. 28, 2001, and claims priority of U.S. patentapplication Ser. No. 09/679,718. The 10/201,652 application was filedJul. 22, 2002, and is (a) a continuation-in part of U.S. patentapplication Ser. No. 10/150,650, and (b) a continuation-in part of the09/679,718 application. The 10/150,650 application was filed May 17,2002, and is a continuation-in-part of the 09/679,718 application. The09/679,718 application was filed Sep. 29, 2000, issued as U.S. Pat. No.6,438,199, and is a continuation-in part of U.S. patent application Ser.No. 09/305,017. The 09/305,017 application was filed May 4, 1999, andissued as U.S. Pat. No. 6,198,793. The present application is alsoclaiming priority of (a) International Application No. PCT/EP00/07258,filed Jul. 28, 2000, (b) German Patent Application No. 299 02 108, filedFeb. 8, 1999, (c) German Patent Application No. 199 03 807, filed Feb.2, 1999, and (d) German Patent Application No. 198 19 898, filed on May5, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns an illumination system for wavelengths ≦193 nm aswell as a projection exposure apparatus with such an illuminationsystem.

2. Description of the Related Art

In order to be able to further reduce the structural widths ofelectronic components, particularly in the submicron range, it isnecessary to reduce the wavelengths of the light utilized formicrolithography. Lithography with very deep UV radiation, so called VUV(Very deep UV) lithography or with soft x-ray radiation, so-called EUV(extreme UV) lithography, is conceivable at wavelengths smaller than 193nm, for example.

An illumination system for a lithographic device, which uses EUVradiation, has been made known from U.S. Pat. No. 5,339,346. For uniformillumination in the reticle plane and filling of the pupil, U.S. Pat.No. 5,339,346 proposes a condenser, which is constructed as a collectorlens and comprises at least 4 pairs of mirror facets, which are arrangedsymmetrically. A plasma light source is used as the light source.

In U.S. Pat. No. 5,737,137, an illumination system with a plasma lightsource comprising a condenser mirror is shown, in which an illuminationof a mask or a reticle to be illuminated is achieved by means ofspherical mirrors.

U.S. Pat. No. 5,361,292 shows an illumination system, in which a plasmalight source is provided, and the point plasma light source is imaged inan annular illuminated surface by means of a condenser, which has fiveaspherical mirrors arranged off-center.

From U.S. Pat. No. 5,581,605, an illumination system has been madeknown, in which a photon beam is split into a multiple number ofsecondary light sources by means of a plate with concave rasterelements. In this way, a homogeneous or uniform illumination is achievedin the reticle plane. The imaging of the reticle on the wafer to beexposed is produced by means of a conventional reduction optics. Agridded mirror is precisely provided with equally curved elements in theillumination beam path. The contents of the above-mentioned patents areincorporated by reference.

EP-A-0 939 341 shows an illumination system and exposure apparatus forilluminating a surface over an illumination field having an arcuateshape with X-ray wave length light. The illumination system comprisesfirst and second optical integrators each with a plurality of reflectingelements. The first and second optical integrators being opposinglyarranged such that a plurality of light source images are formed at theplurality of reflecting elements of the second optical integrator. Toform an arcuate shaped illumination field in the field plane accordingto EP-A-0 939 341 the reflecting elements of the first opticalintegrator have an arcuate shape similar to the arcuate illuminationfield. Such reflecting elements are complicate to manufacture.

EP-A-1 026 547 also shows an illumination system with two opticalintegrators. Similar to the system of EP-A-0 939 341 the reflectingelements of the first optical integrator have an arcuate shape forforming an arcuate shaped illumination field in the field plane.

In EP-A-0 955 641 a system with two optical integrators is shown. Eachof said optical integrators comprises a plurality of raster-elements.The raster elements of the first optical integrator are of rectangularshape. The arc-shaped field in the field plane is formed by at least onegrazing incidence field mirror. The content of the above mentionedpatent-application is incorporated by reference. All above mentionedillumination systems have the disadvantage that the track-length of theillumination system is large.

It is therefore an object of the invention to overcome the disadvantagesof the illumination systems according to the state of the art, toprovide an illumination system for microlithography that fulfills therequirements for advanced lithography with wavelength less or equal to193 nm and which is of compact size.

SUMMARY OF THE INVENTION

The object of the invention is solved by an illumination system with thefeatures of claim 1 and by an projection exposure apparatus according toclaim 17.

The system illuminates a structured reticle arranged in the image planeof the illumination system, which will be imaged by a projectionobjective onto a light sensitive substrate. In reflective lithographysystems the reticle is illuminated with an arc-shaped field, wherein apregiven uniformity of the scanning energy distribution inside the fieldis required, for example better than ±5%. The scanning energy is definedas the line integral over the light intensity in the scanning direction.A further requirement is the illumination of the exit pupil of theillumination system, which is located at the entrance pupil of theprojection objective. A nearly field-independent illumination of theexit pupil is required.

Typical light sources for wavelengths between 100 nm and 200 nm areexcimer lasers, for example an ArF-Laser for 193 nm, an F₂-Laser for 157nm, an Ar₂-Laser for 126 nm and an NeF-Laser for 109 nm. For systems inthis wavelength region refractive components of SiO₂, CaF₂, BaF₂ orother crystallites are used. Since the transmission of the opticalmaterials deteriorates with decreasing wavelength, the illuminationsystems are designed with a combination of refractive and reflectivecomponents. For wavelengths in the EUV wavelength region, between 10 nmand 20 nm, the projection exposure apparatus is designed asall-reflective. A typical EUV light source is aLaser-Produced-Plasma-source, a Pinch-Plasma-Source, a Wiggler-Source oran Undulator-Source.

The light of this primary light source is directed to a first opticalelement, wherein the first optical element is part of a first opticalcomponent. The first optical element is organized as a plurality offirst raster elements and transforms the primary light source into aplurality of secondary light sources. Each first raster elementcorresponds to one secondary light source and focuses an incoming raybundle, defined by all rays intersecting the first raster element, tothe corresponding secondary light source. The secondary light sourcesare arranged in a pupil plane of the illumination system or nearby thisplane. A second optical component is arranged between the pupil planeand the image plane of the illumination system to image the secondarylight sources into an exit pupil of the illumination system, whichcorresponds to the entrance pupil of a following projection objective.The images of the secondary light sources in the exit pupil of theillumination system are therefore called tertiary light sources.

The first raster elements are imaged into the image plane, wherein theirimages are at least partially superimposed on a field that must beilluminated. Therefore, they are known as field raster elements or fieldhoneycombs.

The field raster elements are preferably rectangular. Rectangular fieldraster elements have the advantage that they can be arranged in rowsbeing displaced against each other. Depending on the field to beilluminated they have a side aspect ratio in the range of 5:1 and 20:1.The length of the rectangular field raster elements is typically between15 mm and 50 mm, the width is between 1 mm and 4 mm.

To illuminate an arc-shaped field in the image plane with rectangularfield raster elements the second optical component of the illuminationsystem comprises a first field mirror for transforming the rectangularimages of the rectangular field raster elements to arc-shaped images.The arc length is typically in the range of 80 mm to 105 mm, the radialwidth in the range of 5 mm to 9 mm. The transformation of therectangular images of the rectangular field raster elements can be doneby conical reflection with the first field mirror being a grazingincidence mirror with negative optical power. In other words, theimaging of the field raster elements is distorted to get the arc-shapedimages, wherein the radius of the arc is determined by the shape of theobject field of the projection objective. The first field mirror ispreferably arranged in front of the image plane of the illuminationsystem, wherein there should be a free working distance. For aconfiguration with a reflective reticle the free working distance has tobe adapted to the fact that the rays traveling from the reticle to theprojection objective are not vignetted by the first field mirror.

The surface of the first field mirror is preferably an off-axis segmentof a rotational symmetric reflective surface, which can be designedaspherical or spherical. The axis of symmetry of the supporting surfacegoes through the vertex of the surface. Therefore a segment around thevertex is called on-axis, wherein each segment of the surfaces whichdoes not include the vertex is called off-axis. The supporting surfacecan be manufactured more easily due to the rotational symmetry. Afterproducing the supporting surface the segment can be cut out withwell-known techniques.

The surface of the first field mirror can also be designed as an on-axissegment of a toroidal reflective surface. Therefore the surface has tobe processed locally, but has the advantage that the surrounding shapecan be produced before surface treatment.

The incidence angles of the incoming rays with respect to the surfacenormals at the points of incidence of the incoming rays on the firstfield mirror are preferably greater than 70°, which results in areflectivity of the first field mirror of more than 80%.

The second optical component comprises a second field mirror withpositive optical power. The first and second field mirror together imagethe secondary light sources or the pupil plane respectively into theexit pupil of the illumination system, which is defined by the entrancepupil of the projection objective. The second field mirror is arrangedbetween the plane with the secondary light sources and the first fieldmirror.

The second field mirror is preferably an off-axis segment of arotational symmetric reflective surface, which can be designedaspherical or spherical, or an on-axis segment of a toroidal reflectivesurface.

The incidence angles of the incoming rays with respect to the surfacenormals at the points of incidence of the incoming rays on the secondfield mirror are preferably lower than 25°. Since the mirrors have to becoated with multilayers for the EUV wavelength region, the divergenceand the incidence angles of the incoming rays are preferably as low aspossible to increase the reflectivity, which should be better than 65%.

To reduce the length of the illumination system the field lens comprisespreferably a third field mirror. The third field mirror is preferablyarranged between the plane with the secondary light sources and thesecond field mirror.

The third field mirror has preferably negative optical power and formstogether with the second and first field mirror an optical telescopesystem having a object plane at the secondary light sources and an imageplane at the exit pupil of the illumination system to image thesecondary light sources into the exit pupil. The pupil plane of thetelescope system is arranged at the image plane of the illuminationsystem. Therefore the ray bundles coming from the secondary lightsources are superimposed in the pupil plane of the telescope system orin the image plane of the illumination system accordingly. The firstfield mirror has mainly the function of forming the arc-shaped field,wherein the telescope system is mainly determined by the negative thirdfield mirror and the positive second field mirror.

In another embodiment the third field mirror has preferably positiveoptical power to generate images of the secondary light sources in aplane between the third and second field mirror, forming tertiary lightsources. The tertiary light sources are imaged with the second fieldmirror and the first field mirror into the exit pupil of theillumination system. The images of the tertiary light sources in theexit pupil of the illumination system are called in this case quaternarylight sources.

Since the plane with the tertiary light sources is arranged conjugatedto the exit pupil, this plane can be used to arrange masking blades tochange the illumination mode or to add transmission filters. Thisposition in the beam path has the advantage to be freely accessible.

The third field mirror is similar to the second field mirror preferablyan off-axis segment of a rotational symmetric reflective surface, whichcan be designed aspherical or spherical, or an on-axis segment of atoroidal reflective surface.

The incidence angles of the incoming rays with respect to the surfacenormals at the points of incidence of the incoming rays on the thirdfield mirror are preferably lower than 25°. With the third field mirrorbeing arranged as a normal incidence mirror the beam path can be foldedagain to reduce the overall size of the illumination system.

To avoid vignetting of the beam path the first, second and third fieldmirrors are preferably arranged in a non-centered system. There is nocommon axis of symmetry for the mirrors. An optical axis can be definedas a connecting line between the centers of the used areas on the fieldmirrors, wherein the optical axis is bent at the field mirrors dependingon the tilt angles of the field mirrors.

It is advantageous to insert a second optical element with second rasterelements in the light path after the first optical element with firstraster elements, wherein one first raster element corresponds to one ofthe second raster elements. In this case deflection angles of the firstraster elements are designed to deflect the ray bundles impinging on thefirst raster elements to the corresponding second raster elements.

The second raster elements are preferably arranged at the secondarylight sources and are designed to image together with the field lens thefirst raster elements or field raster elements into the image plane ofthe illumination system, wherein the images of the field raster elementsare at least partially superimposed. The second raster elements arecalled pupil raster elements or pupil honeycombs.

With the tilt angles of the reflective components of the illuminationsystem the beam paths between the components can be bent. Therefore theorientation of the beam cone emitted by the source and the orientationof the image plane system can be arranged according to the requirementsof the overall system. A preferable configuration has a source emittinga beam cone in one direction and an image plane having a surface normalwhich is oriented almost perpendicular to this direction. In oneembodiment the source emits horizontally and the image plane has avertical surface normal. Some light sources like undulator or wigglersources emit only in the horizontal plane. On the other hand the reticleshould be arranged horizontally for gravity reasons. The beam paththerefore has to be bent between the light source and the image planeabout almost 90°. Since mirrors with incidence angles between 30° and60° lead to polarization effects and therefore to light losses, the beambending has to be done only with grazing incidence or normal incidencemirrors. For efficiency reasons the number of mirrors has to be as smallas possible.

By definition all rays intersecting the field in the image plane have togo through the exit pupil of the illumination system. The position ofthe field and the position of the exit pupil are defined by the objectfield and the entrance pupil of the projection objective. For someprojection objectives being centered systems the object field isarranged off-axis of an optical axis, wherein the entrance pupil isarranged on-axis in a finite distance to the object plane. For theseprojection objectives an angle between a straight line from the centerof the object field to the center of the entrance pupil and the surfacenormal of the object plane can be defined. This angle is in the range of3° to 10° for EUV projection objectives. Therefore the components of theillumination system have to be configured and arranged in such a waythat all rays intersecting the object field of the projection objectiveare going through the entrance pupil of the projection objective beingdecentered to the object field. For projection exposure apparatus with areflective reticle all rays intersecting the reticle needs to haveincidence angles greater than 0° to avoid vignetting of the reflectedrays at components of the illumination system.

In the EUV wavelength region all components are reflective components,which are arranged preferably in such a way, that all incidence angleson the components are lower than 25° or greater than 65°. Thereforepolarization effects arising for incidence angles around an angle of 45°are minimized. Since grazing incidence mirrors have a reflectivitygreater than 80%, they are preferable in the optical design incomparison to normal incidence mirrors with a reflectivity greater than65%.

The illumination system is typically arranged in a mechanical box. Byfolding the beam path with mirrors the overall size of the box can bereduced. This box preferably does not interfere with the image plane, inwhich the reticle and the reticle supporting system are arranged.Therefore it is advantageous to arrange and tilt the reflectivecomponents in such a way that all components are completely arranged onone side of the reticle. This can be achieved if the field lenscomprises only an even number of normal incidence mirrors.

The illumination system as described before can be used preferably in aprojection exposure apparatus comprising the illumination system, areticle arranged in the image plane of the illumination system and aprojection objective to image the reticle onto a wafer arranged in theimage plane of the projection objective. Both, reticle and wafer arearranged on a support unit, which allows the exchange or scan of thereticle or wafer.

The projection objective can be a catadioptric lens, as known from U.S.Pat. No. 5,402,267 for wavelengths in the range between 100 nm and 200nm. These systems have typically a transmission reticle.

For the EUV wavelength range the projection objectives are preferablyall-reflective systems with four to eight mirrors as known for examplefrom U.S. Ser. No. 09/503640 showing a six mirror projection lens. Thesesystems have typically a reflective reticle.

For systems with a reflective reticle the illumination beam path betweenthe light source and the reticle and the projection beam path betweenthe reticle and the wafer preferably interfere only nearby the reticle,where the incoming and reflected rays for adjacent object points aretraveling in the same region. If there are no further crossing of theillumination and projection beam path it is possible to separate theillumination system and the projection objective except for the reticleregion.

The projection objective has preferably a projection beam path betweensaid reticle and the first imaging element which is tilted toward theoptical axis of the projection objective. Especially for a projectionexposure apparatus with a reflective reticle the separation of theillumination system and the projection objective is easier to achieve.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below on the basis of the followingdrawings.

FIG. 1: A schematic view of a first embodiment with convex mirrors asfield raster elements and planar mirrors as pupil raster elements

FIG. 2: A schematic view of a second embodiment with convex mirrors asfield raster elements and concave mirrors as pupil raster elements.

FIG. 3: A schematic view of the principal setup of an illuminationsystem.

FIG. 4: An Arrangement of the field raster elements.

FIG. 5: An Arrangement of the pupil raster elements.

FIG. 6: A schematic view of a third embodiment with a concavepupil-imaging field mirror and a convex field-forming field mirror.

FIG. 7: A schematic view of a further embodiment with a second opticalcomponent comprising a telescope system and a convex field-forming fieldmirror.

FIG. 8: A detailed view of the embodiment of FIG. 7.

FIG. 9: Intensity distribution of the embodiment of FIG. 7.

FIG. 10: Illumination of the exit pupil of the illumination system ofthe embodiment of FIG. 7.

FIG. 11: A schematic view of a embodiment with an intermediate image ofthe light source.

FIG. 12: A detailed view of a projection exposure apparatus.

DESCRIPTION OF THE INVENTION

FIG. 1 shows an first embodiment of the invention in a schematicallyview. The beam cone of the light source 7001 is collected by theellipsoidal collector mirror 7003 and is directed to the plate with thefield raster elements 7009. The collector mirror 7003 is designed togenerate an image 7005 of the light source 7001 between the plate withthe field raster elements 7009 and the plate with the pupil rasterelements 7015 if the plate with the field raster elements 7009 would bea planar mirror as indicated by the dashed lines. The convex fieldraster elements 7009 are designed to generate point-like secondary lightsources 7007 at the pupil raster elements 7015, since the light source7001 is also point-like. Therefore the pupil raster elements 7015 aredesigned as planar mirrors. Since the intensity at the point-likesecondary light sources 7007 is very high, the planar pupil rasterelements 7015 can alternatively be arranged defocused from the secondarylight sources 7007. The distance between the secondary light sources7007 and the pupil raster elements 7015 should not exceed 20% of thedistance between the field raster elements and the pupil rasterelements. The pupil raster elements 7015 are tilted to superimpose theimages of the field raster elements 7009 together with the field lens7021 formed as the field mirrors 7023 and 7027 in the field 7031 to beilluminated. Both, the field raster elements 7009 and the pupil rasterelements 7015 are tilted. Therefore the assignment between the fieldraster elements 7009 and pupil raster elements 7015 is defined by theuser. In the embodiment of FIG. 1 the field raster elements at thecenter of the plate with the field raster elements 7009 correspond tothe pupil raster elements at the border of the plate with the pupilraster elements 7015 and vice versa. The tilt angles and the tilt axesof the field raster elements are determined by the directions of theincoming ray bundles and by the positions of the corresponding pupilraster elements 7015. Since for each field raster element 7009 the tiltangle and the tilt axis is different, also the planes of incidencedefined by the incoming and reflected centroid rays are not parallel.The tilt angles and the tilt axes of the pupil raster elements 7015 aredetermined by the positions of the corresponding field raster elements7009 and the requirement that the images of the field raster elements7009 have to be superimposed in the field 7031 to be illuminated. Theconcave field mirror 7023 images the secondary light sources 7007 intothe exit pupil 7033 of the illumination system forming tertiary lightsources 7035, wherein the convex field mirror 7027 being arranged atgrazing incidence transforms the rectangular images of the rectangularfield raster elements 7009 into arc-shaped images.

FIG. 2 shows second embodiment in a schematically view. Correspondingelements have the same reference numbers as those in FIG. 1 increased by100. Therefore, the description to these elements is found in thedescription to FIG. 1. In this embodiment the light source 7101 andtherefore also the secondary light sources 7107 are extended. The pupilraster elements 7115 are designed as concave mirrors to image the fieldraster elements 7109 into the image plane 7129. It is also possible toarrange the pupil raster elements 7115 not at the secondary lightsources 7107, but defocused. The influence of the defocus on the imagingof the field raster elements 7109 has to be considered in the opticalpower of the pupil raster elements.

FIG. 3 shows in a schematic view the imaging of one field raster element7209 into the reticle plane 7229 forming an image 7231 and the imagingof the corresponding secondary light source 7207 into the exit pupil7233 of the illumination system forming a tertiary light source 7235.Corresponding elements have the same reference numbers as those in FIG.1 increased by 200. Therefore, the description to these elements isfound in the description to FIG. 1.

The field raster elements 7209 are rectangular and have a length X_(FRE)and a width Y_(FRE). All field raster elements 7209 are arranged on anearly circular plate with a diameter D_(FRE). They are imaged into theimage plane 7229 and superimposed on a field 7231 with a lengthX_(field) and a width Y_(fleld), wherein the maximum aperture in theimage plane 7229 is denoted by NA_(field). The field size corresponds tothe size of the object field of the projection objective, for which theillumination system is adapted to.

The plate with the pupil raster elements 7215 is arranged in a distanceof Z₃ from the plate with the field raster elements 7209. The shape ofthe pupil raster elements 7215 depends on the shape of the secondarylight sources 7207. For circular secondary light sources 7207 the pupilraster elements 7215 are circular or hexagonal for a dense packaging ofthe pupil raster elements 7215. The diameter of the plate with the pupilraster elements 7215 is denoted by D_(PRE).

The pupil raster elements 7215 are imaged by the second opticalcomponent, which is depicted in FIG. 3 as a field lens 7221 into theexit pupil 7233 having a diameter of D_(EP). The distance between theimage plane 7229 of the illumination system and the exit pupil 7233 isdenoted with Z_(EP). Since the exit pupil 7233 of the illuminationsystem corresponds to the entrance pupil of the projection objective,the distance Z_(EP) and the diameter D_(EP) are predetermined values.The entrance pupil of the projection objective is typically illuminatedup to a user-defined filling ratio σ.

The data for a preliminary design of the illumination system can becalculated with the equations and data given below. The values for theparameters are typical for a EUV projection exposure apparatus. Butthere is no limitation to these values. Wherein the schematic design isshown for a refractive linear system it can be easily adapted forreflective systems by exchanging the lenses with mirrors.

The field 7231 to be illuminated is defined by a segment of an annulus.The Radius of the annulus is

R_(fleld)=138 mm.

The length and the width of the segment are

X_(field)=88 mm, Y_(field)=8 mm

Without the field-forming field mirror of the second optical componentwhich transforms the rectangular images of the field raster elementsinto arc-shaped images the field to be illuminated is rectangular withthe length and width defined by the segment of the annulus.

The distance from the image plane to the exit pupil is

Z_(EP)=1320 mm.

The object field of the projection objective is an off-axis field. Thedistance between the center of the field and the optical axis of theprojection objective is given by the radius R_(field). Therefore theincidence angle of the centroid ray in the center of the field is 6°.

The aperture at the image plane of the projection objective isNA_(wafer) _(=0.25). For a reduction projection objective with amagnification ratio of β_(proj)=−0.25 and a filling ratio of σ=0.8 theaperture at the image plane of the illumination system is${NA}_{field} = {{\sigma \cdot \frac{{NA}_{wafer}}{4}} = 0.05}$  D_(EP)=2 tan|arcsin(NA _(field))|·Z _(EP)≈2NA _(EP) ·Z _(EP)≈132 mm

The distance Z₃ between the field raster elements and the pupil rasterelements is related to the distance Z_(EP) between the image plane andthe exit pupil by the depth magnification α:Z _(EP) =α·Z ₃

The size of the field raster elements is related to the field size bythe lateral magnification β_(field:)X _(field)=β_(field) ·X _(FRE)Y _(field)=β_(field) ·Y _(FRE)

The diameter D_(PRE) of the plate with the pupil raster elements and thediameter D_(EP) of the exit pupil are related by the lateralmagnification β_(pupil:)D _(EP)=β_(pupil) ·D _(PRE)

The depth magnification α is defined by the product of the lateralmagnifications β_(field) and β_(pupil:)α=β_(field)·β_(pupil)

The number of raster elements being superimposed at the field is set to200.

With this high number of superimposed images the required fieldillumination uniformity can be achieved.

Another requirement is to minimize the incidence angles on thecomponents. For a reflective system the beam path is bent at the platewith the field raster elements and at the plate with the pupil rasterelements. The bending angles and therefore the incidence angles areminimum for equal diameters of the two plates:D_(PRE)=D_(FRE)${200 \cdot X_{PRE} \cdot Y_{PRE}} = {{200 \cdot \frac{X_{field} \cdot Y_{field}}{\beta_{field}^{2}}} = {\frac{D_{EP}^{2}}{\beta_{pupil}^{2}} = {\frac{\beta_{field}^{2}}{\alpha^{2}}D_{EP}^{2}}}}$

The distance Z₃ is set to Z₃=900 mm. This distance is a compromisebetween low incidence angles and a reduced overall length of theillumination system. $\alpha = {\frac{Z_{EP}}{Z_{3}} = 1.47}$Therefore${\beta_{field}} \approx \sqrt[4]{\frac{200 \cdot X_{field} \cdot Y_{field}}{D_{EP}^{2}}\alpha^{2}} \approx 2.05$${\beta_{pupil}} \approx \frac{\alpha}{\beta_{field}} \approx 0.7$$D_{FRE} = {D_{PRE} = {{\frac{\beta_{field}}{\alpha}D_{EP}} \approx {200\quad{mm}}}}$$X_{FRE} = {\frac{X_{field}}{\beta_{field}} \approx {43\quad{mm}}}$$Y_{FRE} = {\frac{Y_{field}}{\beta_{field}} \approx {4\quad{mm}}}$With these values the principal layout of the illumination system isknown.

In a next step the field raster elements 7309 have to be distributed onthe plate as shown in FIG. 4. The two-dimensional arrangement of thefield raster elements 7309 is optimized for efficiency. Therefore thedistance between the field raster elements 7309 is as small as possible.Field raster elements 7309, which are only partially illuminated, willlead to uniformity errors of the intensity distribution in the imageplane, especially in the case of a restricted number of field rasterelements 7309. Therefore only these field raster elements 7309 areimaged into the image plane which are illuminated almost completely.FIG. 4 shows a possible arrangement of 216 field raster elements 7309.The solid line 7339 represents the border of the circular illuminationof the plate with the field raster elements 7309. Therefore the fillingefficiency is approximately 90%. The rectangular field raster elements7309 have a length X_(FRE)=46.0 mm and a width Y_(FRE)=2.8 mm. All fieldraster elements 7309 are inside the circle 7339 with a diameter of 200mm. The field raster elements 7309 are arranged in 69 rows 7341 beingarranged one among another. The field raster elements 7309 in the rows7341 are attached at the smaller y-side of the field raster elements7309. The rows 7341 consist of one, two, three or four field rasterelements 7309. Some rows 7341 are displaced relative to the adjacentrows 7341 to distribute the field raster elements 7309 inside the circle7339. The distribution is symmetrical to the y-axis.

FIG. 5 shows the arrangement of the pupil raster elements 7415. They arearranged on a distorted grid to compensate for distortion errors of thefield lens. If this distorted grid of pupil raster elements 7415 isimaged into the exit pupil of the illumination system by the field lensan undistorted regular grid of tertiary light sources will be generated.The pupil raster elements 7415 are arranged on curved lines 7443 tocompensate the distortion introduced by the field-forming field mirror.The distance between adjacent pupil raster elements 7415 is increased iny-direction to compensate the distortion introduced by field mirrorsbeing tilted about the x-axis. Therefore the pupil raster elements 7415are not arranged inside a circle. The size of the pupil raster elements7415 depends on the source size or source étendue. If the source étendueis much smaller than the required étendue in the image plane, thesecondary light sources will not fill the plate with the pupil rasterelements 7415 completely. In this case the pupil raster elements 7415need only to cover the area of the secondary light sources plus someoverlay to compensate for source movements and imaging aberrations ofthe collector-field raster element unit. In FIG. 5 circular pupil rasterelements 7415 are shown.

Each field raster element 7309 correspond to one of the pupil rasterelements 7415 according to a assignment table and is tilted to deflectan incoming ray bundle to the corresponding pupil raster element 7415. Aray coming from the center of the light source and intersecting thefield raster element 7309 at its center is deflected to intersect thecenter of the corresponding pupil raster element 7415. The tilt angleand tilt axis of the pupil raster element 7415 is designed to deflectthis ray in such a way, that the ray intersects the field in its center.

The second optical component comprising the field mirror images theplate with the pupil raster elements into the exit pupil and generatesthe arc-shaped field with the desired radius R_(field). ForR_(field)=138 mm, the field forming gracing incidence field mirror hasonly low negative optical power. The optical power of the field-formingfield mirror has to be negative to get the correct orientation of thearc-shaped field. Since the magnification ratio of the second opticalcomponent has to be positive, another field mirror with positive opticalpower is required. The field mirror with positive optical power is anormal incidence mirror. The usage of a normal incidence mirror providesfor a compact size of the illumination system.

FIG. 6 shows a schematic view of a embodiment comprising a light source7501, a collector mirror 7503 , a plate with the field raster elements7509, a plate with the pupil raster elements 7515, a field lens 7521, aimage plane 7529 and a exit pupil 7535. The field lens 7521 has onenormal-incidence mirror 7523 with positive optical power for pupilimaging and one grazing-incidence mirror 7527 with negative opticalpower for field shaping. Exemplary for the imaging of all secondarylight sources, the imaging of one secondary light source 7507 into theexit pupil 7533 forming a tertiary light source 7535 is shown. Theoptical axis 7545 of the illumination system is not a straight line butis defined by the connection lines between the single components beingintersected by the optical axis 7545 at the centers of the components.Therefore, the illumination system is a non-centered system having anoptical axis 7545 being bent at each component to get a beam path freeof vignetting. There is no common axis of symmetry for the opticalcomponents. Projection objectives for EUV exposure apparatus aretypically centered systems with a straight optical axis and with anoff-axis object field. The optical axis 7547 of the projection objectiveis shown as a dashed line. The distance between the center of the field7531 and the optical axis 7547 of the projection objective is equal tothe field radius R_(field).

In another embodiment as shown in FIG. 7, a telescope objective in thefield lens 7621 comprising the field mirror 7623 with positive opticalpower, the field mirror 7625 with negative optical power and the fieldmirror 7627 is applied to reduce the track length. Correspondingelements have the same reference numbers as those in FIG. 6 increased by100. Therefore, the description to these elements is found in thedescription to FIG. 6. The field mirror 7625 and the field mirror 7623of the telescope objective in FIG. 5 are formed as an off-axisCassegrainian configuration. The telescope objective has an object planeat the secondary light sources 7607 and an image plane at the exit pupil7633 of the illumination system. The pupil plane of the telescopeobjective is arranged at the image plane 7629 of the illuminationsystem. In this configuration, having five normal-incidence reflectionsat the mirrors 7603, 7609, 7615, 7625 and 7623 and one grazing-incidencereflection at the mirror 7627, all mirrors are arranged below the imageplane 7629 of the illumination system. Therefore, there is enough spaceto install the reticle and the reticle support system.

In FIG. 8 a detailed view of the embodiment of FIG. 7 is shown.Corresponding elements have the same reference numbers as those in FIG.7 increased by 100. Therefore, the description to these elements isfound in the description to FIG. 7. The components are shown in ay-z-sectional view, wherein for each component the local co-ordinatesystem with the y- and z-axis is shown. For the collector mirror 7703and the field mirrors 7723, 7725 and 7727 the local co-ordinate systemsare defined at the vertices of the mirrors. For the two plates with theraster elements the local co-ordinate systems are defined at the centersof the plates. In table 2 the arrangement of the local co-ordinatesystems with respect to the local co-ordinate system of the light source7701 is given. The tilt angles α,β and γ about the x-, y- and z-axis aredefined in a right-handed system.

TABLE 2 Co-ordinate systems of vertices of mirrors X [mm] Y [mm] Z [mm]α [°] β [°] γ [°] Light source 7701 0.0 0.0 0.0 0.0 0.0 0.0 Collectormirror 0.0 0.0 125.0 0.0 0.0 0.0 7703 Plate with field 0.0 0.0 −975.010.5 180.0 0.0 raster elements 7709 Plate with pupil 0.0 −322.5 −134.813.5 0.0 180.0 raster elements 7715 Field mirror 7725 0.0 508.4 −1836.1−67.8 0.0 180.0 Field mirror 7723 0.0 204.8 −989.7 −19.7 0.0 180.0 Fieldmirror 7727 0.0 −163.2 −2106.2 49.4 180.0 0.0 Image plane 7731 0.0−132.1 −1820.2 45.0 0.0 0.0 Exit pupil 7733 0.0 −1158.1 −989.4 45.0 0.00.0

The surface data are given in table 3. The radius R and the conicalconstant K define the surface shape of the mirrors according to theformula${z = {\frac{\rho\quad h^{2}}{1 + \sqrt{1 - {\left( {1 + \kappa} \right)\rho^{2}h^{2}}}} + {\sum\limits_{k = 1}\quad{c_{k}h^{{2k} + 2}}}}},$wherein h is the radial distance of a surface point from the z-axis.

TABLE 3 Optical data of the components Field Pupil Collector rasterraster Field Field Field mirror element element mirror mirror mirror7703 7709 7715 7725 7723 7727 R [mm] −235.3 ∞ −1239.7 −534.7 −937.7−65.5 K −0.77855 0.0 0.0 −0.0435 −0.0378 −1.1186 Focal — ∞ 617.6 −279.4477.0 −757.1 length f [mm]

The light source 7701 in this embodiment is a Laser-Produced-Plasmasource having a diameter of approximately 0.3 mm generating a beam conewith an opening angle of 83°. To decrease the contamination of thecollector mirror 7703 by debris of the source 7701 the distance to thecollector mirror 7703 is set to 125 mm.

The collector mirror 7703 is an elliptical mirror, wherein the lightsource 7701 is arranged in the first focal point of the ellipsoid andwherein the plate with the pupil raster elements 7715 is arranged in thesecond focal point of the ellipsoid.

Therefore the field raster elements 7709 can be designed as planarmirrors. The distance between the vertex of the collector mirror 7703and the center of the plate with the field raster elements 7709 is 1100mm. The field raster elements 7709 are rectangular with a lengthX_(FRE)=46.0 mm and a width Y_(FRE)=2.8 mm. The arrangement of the fieldraster elements is shown in FIG. 4. The tilt angles and tilt axis aredifferent for each field raster element 7709, wherein the field rasterelements are tilted to direct the incoming ray bundles to thecorresponding pupil raster elements 7715. The tilt angles are in therange of −4° to 4°. The mean incidence angle of the rays on the fieldraster elements is 10.5°. Therefore the field raster elements 7709 areused at normal incidence.

The plate with the pupil raster elements 7715 is arranged in a distanceof 900 mm from the plate with the field raster elements 7709. The pupilraster elements 7715 are concave mirrors. The arrangement of the pupilraster elements 7715 is shown in FIG. 5. The tilt angles and tilt axisare different for each pupil raster element 7715, wherein the pupilraster elements 7715 are tilted to superimpose the images of the fieldraster elements 7709 in the image plane 7731. The tilt angles are in therange of −4° to 4°. The mean incidence angle of the rays on the pupilraster elements 7715 is 7.5°. Therefore the pupil raster elements 7715are used at normal incidence.

The field mirror 7725 is a convex mirror. The used area of this mirrordefined by the incoming rays is an off-axis segment of a rotationalsymmetric conic surface. The mirror surface is drawn in FIG. 6 from thevertex up to the used area as dashed line. The distance between thecenter of the plate with the pupil raster elements 7715 and the centerof the used area on the field mirror 7725 is 1400 mm. The mean incidenceangle of the rays on the field mirror 7725 is 12°. Therefore the fieldmirror 7725 is used at normal incidence.

The field mirror 7723 is a concave mirror. The used area of this mirrordefined by the incoming rays is an off-axis segment of a rotationalsymmetric conical surface. The mirror surface is drawn in FIG. 75 fromthe vertex up to the used area as dashed line. The distance between thecenter of the used area on the field mirror 7725 and the center of theused area on the field mirror 7723 is 600 mm. The mean incidence angleof the rays on the field mirror 7723 is 7.5°. Therefore the field mirror7723 is used at normal incidence.

The field mirror 7727 is a convex mirror. The used area of this mirrordefined by the incoming rays is an off-axis segment of a rotationalsymmetric conic surface. The mirror surface is drawn in FIG. 6 from thevertex up to the used area as dashed line. The distance between thecenter of the used area on the field mirror 7723 and the center of theused area on the field mirror 7727 is 600 mm. The mean incidence angleof the rays on the field mirror 7727 is 78°. Therefore the field mirror7727 is used at grazing incidence. The distance between the field mirror7727 and the image plane 7731 is 300 mm.

In another embodiment the field mirror and the field mirror are replacedwith on-axis toroidal mirrors. The vertices of these mirrors arearranged in the centers of the used areas. The convex field mirror has aradius R_(y)=571.3 mm in the y-z-section and a radius R_(x)=546.6 mm inthe x-z-section. This mirror is tilted about the local x-axis about 12°to the local optical axis 7745 defined as the connection lines betweenthe centers of the used areas of the mirrors. The concave field mirrorhas a radius R_(y)=−962.14 mm in the y-z-section and a radiusR_(x)=−945.75 mm in the x-z-section. This mirror is tilted about thelocal x-axis about 7.5° to the local optical axis 7745.

FIG. 9 shows the illuminated arc-shaped area in the image plane 7731 ofthe illumination system presented in FIG. 8. The orientation of they-axis is defined in FIG. 8. The solid line 7849 represents the50%-value of the intensity distribution, the dashed line 7851 the10%-value. The width of the illuminated area in y-direction is constantover the field. The intensity distribution is the result of a simulationdone with the optical system given in table 2 and table 3.

FIG. 10 shows the illumination of the exit pupil 7733 for an objectpoint in the center (x=0 mm; y=0 mm) of the illuminated field in theimage plane 7731. The arrangement of the tertiary light sources 7935corresponds to the arrangement of the pupil raster elements 7715, whichis presented in FIG. 5. Wherein the pupil raster elements in FIG. 5 arearranged on a distorted grid, the tertiary light sources 7935 arearranged on a undistorted regular grid. It is obvious in FIG. 10, thatthe distortion errors of the imaging of the secondary light sources dueto the tilted field mirrors and the field-shaping field mirror arecompensated. The shape of the tertiary light sources 7935 is notcircular, since the light distribution in the exit pupil 7733 is theresult of a simulation with a Laser-Plasma-Source which was notspherical but ellipsoidal. The source ellipsoid was oriented in thedirection of the local optical axis. Therefore also the tertiary lightsources are not circular, but elliptical. Due to the mixing of the lightchannels and the user-defined assignment between the field rasterelements and the pupil raster elements, the orientation of the tertiarylight sources 7935 is different for each tertiary light source 7935.

Due to the mixing of the light channels and the user-defined assignmentbetween the field raster elements and the pupil raster elements, theorientation of the tertiary light sources 7935 is different for nearbyeach tertiary light source 7935. Therefore, the planes of incidence ofat least two field raster elements have to intersect each other. Theplane of incidence of a field raster element is defined by the centroidray of the incoming bundle and its corresponding deflected ray.

FIG. 11 shows another embodiment in a schematic view. Correspondingelements have the same reference numbers as those in FIG. 6 increased by800. Therefore, the description to these elements is found in thedescription to FIG. 6. In this embodiment the collector mirror 8303 isdesigned to generate an intermediate image 8361 of the light source 8301in front of the plate with the field raster elements 8309. Nearby thisintermediate image 8363 a transmission plate 8365 is arranged. Thedistance between the intermediate image 8363 and the transmission plate8365 is so large that the plate 8365 will not be destroyed by the highintensity near the intermediate focus. The distance is limited by themaximum diameter of the transmission plate 8365 which is in the order of200 mm. The maximum diameter is determined by the possibility tomanufacture a plate being transparent at EUV. The transmission plate8365 can also be used as a spectral purity filter to select the usedwavelength range. Instead of the absorptive transmission plate 8365 alsoa reflective grating filter can be used. The plate with the field rasterelements 8309 is illuminated with a diverging ray bundle. Since the tiltangles of the field raster elements 8309 are adjusted according to acollecting surface the diverging beam path can be transformed to anearly parallel one. Additionally, the field raster elements 8309 aretilted to deflect the incoming ray bundles to the corresponding pupilraster elements 8315.

FIG. 12 shows an EUV projection exposure apparatus in a detailed view.The illumination system is the same as shown in detail in FIG. 8.Corresponding elements have the same reference numbers as those in FIG.8 increased by 700. Therefore, the description to these elements isfound in the description to FIG. 8. In the image plane 8429 of theillumination system the reticle 8467 is arranged. The reticle 8467 ispositioned by a support system 8469. The projection objective 8471having six mirrors images the reticle 8467 onto the wafer 8473 which isalso positioned by a support system 8475. The mirrors of the projectionobjective 8471 are centered on a common straight optical axis 8447. Thearc-shaped object field is arranged off-axis. The direction of the beampath between the reticle 8467 and the first mirror 8477 of theprojection objective 8471 is tilted to the optical axis 8447 of theprojection objective 8471. The angles of the chief rays 8479 withrespect to the normal of the reticle 8467 are between 5° and 7°. Asshown in FIG. 80 the illumination system 8479 is well separated from theprojection objective 8471. The illumination and the projection beam pathinterfere only nearby the reticle 8467. The beam path of theillumination system is folded with reflection angles lower than 25° orhigher than 75° in such a way that the components of the illuminationsystem are arranged between the plane 8481 with the reticle 8467 and theplane 8383 with the wafer 8473.

1. Illumination system, particularly for microlithography with wavelengths ≦193 nm, comprising: a primary light source; a first optical component; a second optical component; an image plane; and an exit pupil, wherein said first optical component transforms said primary light source into a plurality of secondary light sources that are imaged by said second optical component in said exit pupil, wherein said first optical component includes a first optical element having a plurality of first raster elements, that are imaged into said image plane, producing a plurality of images being superimposed at least partially on a field in said image plane, wherein said plurality of first raster elements are rectangular, wherein said field is a segment of an annulus, wherein said second optical component includes a first field mirror with negative optical power for shaping said field to said segment of said annulus and a second field mirror with positive optical power, wherein each of a plurality of rays intersects said first field mirror with an incidence angle greater than 70°, and wherein each of said plurality of rays intersects said second field mirror with an incidence angle of less than 25°.
 2. The illumination system according to claim 1, wherein said first field mirror is an off-axis segment of a rotational symmetric reflective surface.
 3. The illumination system according to claim 1, wherein said first field mirror is an on-axis segment of a toroidal reflective surface.
 4. The illumination system according to claim 1, wherein said second field mirror is an off-axis segment of a rotational symmetric reflective surface.
 5. The illumination system according to claim 1, wherein said second field mirror is an on-axis segment of a toroidal reflective surface.
 6. The illumination system according to claim 1, wherein said second optical component comprises a third field mirror.
 7. The illumination system according to claim 6, wherein said third mirror has negative optical power.
 8. The illumination system according to claim 6, wherein said first, second and third field mirrors form (a) a telescope-objective with a tele-object plane at said plurality of secondary light sources, (b) a tele-pupil plane at said image plane of said illumination system, and (c) a tele-image plane at said exit pupil.
 9. The illumination system according to claim 6, wherein each of a plurality of rays intersects said third field mirror with an incidence angle less than 25°.
 10. The illumination system according to claim 6, wherein said third field mirror is an off-axis segment of a rotational symmetric reflective surface.
 11. The illumination system according to claim 6, wherein said third field mirror is an on-axis segment of a toroidal reflective surface.
 12. The illumination system according to claim 6, wherein said first, second and third field mirrors are forming a non-centered system.
 13. The illumination system according to claim 1, wherein said second optical component comprises an even number of normal incidence mirrors having incidence angles of less than 25°.
 14. The illumination system according to claim 1, wherein said first optical component further comprises a second optical element having a plurality of second raster elements, wherein each of said plurality of first raster elements corresponds to one of said plurality of second raster elements, and wherein each of said plurality of first raster elements deflects an incoming ray bundle to said corresponding one of said plurality of second raster elements.
 15. The illumination system according to claim 14, wherein said plurality of second raster elements and said second optical component image said corresponding plurality of first raster elements into said image plane.
 16. The illumination system according to claim 14, wherein said plurality of second raster elements are concave mirrors.
 17. A projection exposure apparatus for microlithography comprising: the illumination system of claim 1; a reticle being located at said image plane; a light-sensitive object on a support system; and a projection objective to image said reticle onto said light-sensitive object. 